Absorbent webs which comprise entangled masses of fibers, i.e., fibrous webs, are well known in the art. Such webs can imbibe liquids, such as discharged body fluids, both by an absorption mechanism wherein fluid is taken up by the fiber material itself and by a wicking mechanism wherein fluid is acquired by, distributed through and stored in the capillary interstices between fibers. One means for improving the absorbent capacity of such fibrous web structures is to incorporate therein a superabsorbent material, such as polymeric gelling material (also referred to as hydrogel-forming material superabsorbent polymers, etc.) which imbibes fluid. The superabsorbent material serves to retain fluid such as discharged body liquids.
Notwithstanding the existence of absorbent cores as described above, there remains a need to provide absorbent cores with improved effective absorbent capacity. One way to theoretically do this would be to increase the level of polymeric gelling material in the absorbent core. Unfortunately, high levels of polymeric gelling material especially levels in excess of about 15%, in fibrous webs typically used in absorbent cores can induce a phenomena referred to as gel-blocking. Gel-blocking occurs when the polymeric gelling material located in regions first contacted with fluid increase in volume as a consequence of imbibing the fluid and forming the hydrogel. When polymeric gelling material concentration is too high, the hydrogel can block additional fluid from reaching other regions of the core having unused absorbent capacity. The occurrence of gel blocking can lead to leakage during usage of the absorbent article.
Polymeric gelling materials have been developed which can exhibit a reduced tendency to result in gel blocking. However, these improved polymeric gelling materials, and other superabsorbent materials, are subject to performance limitations of the web of cellulosic fibers in which particles of gelling material are distributed. In particular, upon initial wetting, the cellulosic fiber webs tend to collapse to a higher density and, consequently, exhibits reduced capacity, permeability, and fluid transport efficiency.
Another reason why many absorbent articles such as catamenial pads, adult incontinent products, and diapers are subject to leakage is inability to absorb second and subsequent discharges of fluid even if the first fluid discharge has been effectively absorbed. Leakage due to second and subsequent discharges is especially prevalent during the night, when users commonly experience multiple discharges before being attended to. One reason for the inability of many absorbent articles to adequately handle multiple discharges of fluid, in addition to the reasons discussed above, is the inability of the absorbent core to transport discharged fluid away from the region of discharge once the absorbent capacity of that region has been reached. Overall performance of the absorbent article is limited by the inability to have the fluid transported to the farthest reaches of the core.
One means which has been used to increase the fluid transport ability of the absorbent core is to create a smaller average pore regime by densification of a conventional core. While this does decrease the overall pore size average, generally the largest cells see the largest ratio of collapse and the smaller pores, which determine ultimate capillary pressure (vertical wicking height), see the least ratio of change. This loss of large pore capacity results in a high loss of capacity and more importantly, fluid permeability.
Another means which has been used in the past to increase the fluid transport ability of the absorbent core is to blend in certain amounts of fine fibers and particles which have a high surface area with chemically stiffened conventional cellulose fibers, non-stiffened cellulosic material, synthetic fibers, chemical additives and thermoplastic polymers. Since surface area per unit volume has a strong influence on the capillary pressure of a particular substrate, these high surface area fibers do provide higher vertical wicking. But the resulting structure tends to be very dense and has low fluid transport capability, or "flux", defined herein as the ability to move a certain amount of fluid through a given cross-section of a material to a particular height in a specific time. The increase in capillary pressure (height) from these fine fibers results in a more significant loss of volume transported to a given height.